How to write a template_preprocess_node hook to get the Image Style for an Image field?

Question

I have an image field on Basic Page content type with a View Mode where format = Image and Image Style = Large.

How can I write a template_preprocess_node hook to get the Image style that is based on the View mode?

Here is what I was thinking but got stuck and cannot figure out what to call.

function mymodule_preprocess_node(&$variables) {
  $variables['view_mode'] = $variables['elements']['#view_mode'];
  $node = $variables['elements']['#node'];
  if($variables['view_mode'] = 'default' && $node->hasField('field_image')){
    $fieldImage = $node->get('field_image');
    /** stuck here as I cannot find method to get the Image Style **/
    //goal
    $image_style = $fieldImage->getImageStyle() // not sure what method this would be
    $image_path = $fieldImage->entity->getFileUri();
    $image_url = ImageStyle::load($image_style)->buildUrl($image_path);
    $variables['content'][0]['image_url'] = $image_url; // this is the goal
  }

}

twig template

<div style="background-image: url('{{ content.image_url }}')"></div> 

Answer 1

It is part of the render array generated by the formatter: https://api.drupal.org/api/drupal/core%21modules%21image%21src%21Plugin%21Field%21FieldFormatter%21ImageFormatter.php/function/ImageFormatter%3A%3AviewElements/8.2.x

You can see that it is set on #image_style, so in node preprocess, that should be $variables['content'][field_imge][0]['#image_style'].

Depending on what you want to do, you could also write your own formatter or you could preprocess/alter the image_formatter template.